A 66-year-old man with chronic B-cell leukemia who had antibody to hepatitis B core antigen (anti-HBc) but not hepatitis B surface antigen (HBsAg) suffered from lethal hepatitis caused by hepatitis B virus (HBV) reactivation. He initially lacked circulating viral genomes in his sera and did not have a past history of liver dysfunction. In this patient, the immunosuppressive condition introduced by disease progression of leukemia induced reactivation of dormant HBV, and the withdrawal of chemotherapy resulted in fatal liver failure. Mutation-specific assay based on competitive polymerase chain reaction (PCR) and sequencing analyses revealed the predominant reactivation of an HBV strain with missence mutation (point mutation G-to-A at nucleotide 1896) in the precore regions, as well as point mutations in the core promoter regions. Therefore, it is important to note the risk of HBV reactivation, with resulting lethal hepatic failure, in anti-HBc-positive healthy individuals, even when they lack HBsAg. who receive immunomodulating therapy.